Probing Charge Carrier Movement in Organic Semiconductor Thin Films via Nanowire Conductance Spectroscopy

Understanding movement of charge carriers within organic semiconductor films is crucial for applications in photovoltaics and flexible electronics. This study theoretically investigates the use silicon nanowires for probing spatial and temporal distribution of charge carriers in organic thin films and estimates the sensitivity of electrical conductance of silicon nanowires to changes of charge states within an organic semiconductor physisorbed on the surface of the nanowire. Elastic scattering caused by motion of charge carriers near the nanowire modifies the mean-free path for backscattering of electrons propagating within it, which we have mathematically expressed in terms of the causal Green's functions. The scattering potential has been computed by using a combination of a polarizable continuum model and density functional theory with a range-separated exchange-correlation functional for organic molecules and the semiempirical tight-binding model for silicon. For a single charge carrier in crystalline tetracene, ultrathin silicon nanowires with characteristic sizes of the cross section below 2 nm produce a detectable conductance change at room temperature. For larger nanowires the sensitivity is reduced; however, the conductance change grows with the number of charged molecules, with sub-4 nm nanowires being sensitive enough to detect several tens of charge carriers. We propose using noise spectroscopy to access the temporal evolution of the charge states. Information regarding the spatial distribution of charge carries in organic thin films can be obtained by using a grid of nanowire resistors and electric impedance tomography.